This invention relates to thermal cyclic oxidation resistant and hot workable alloys. More particularly, the invention relates to iron-chromium-aluminum alloys with rare earth additions, particularly cerium and lanthanum.
It is known to provide iron-chromium-aluminum alloys having additions of yttrium for the purpose of high temperature oxidation resistance and improved oxide surfaces. U.S. Pat. No. 3,027,252, issued May 27, 1962, disclosed at 25-95% chromium, 0.5-4% aluminum and 0.5-3% yttrium alloy for high temperature oxidation resistance at greater than 2000.degree. F. (1094.degree. C.). An objective of the alloy was to provide improved workability and a thermal shock resistant and non-spalling oxide film. Another patent No. 3,298,826, issued Jan. 17, 1967, has as its objective to improve the resistance to embrittlement and hardening of the alloys between 650-1300.degree. F. (343-704.degree. C.) while retaining the oxidation and corrosion resistance. The patent discloses that embrittlement is avoided by lowering the chromium content below 15%. U.S. Pat. No. 4,230,489, issued Oct. 28, 1980, relates to the addition of 1 to 2% silicon to such alloys for increasing the corrosion resistance.
Generally, such alloys have properties which are useful in high temperature environments which require oxidation resistance and it has been proposed that they may be useful as a substrate material such as for catalytic converters, as well as for resistance heating elements and radiant heating elements in gas or oil stoves. As a catalytic substrate, a metallic substrate offers many advantages over present ceramic substrates. For example, a metal substrate is substantially more shock resistant and vibration resistant, as well as having a greater thermal conductivity, than ceramic. Furthermore, a metallic substrate can be more easily fabricated into thin foil and fine honeycomb configurations to provide greater surface area and lighter weight.
Present iron-chromium-aluminum alloys containing yttrium may provide some satisfactory properties of oxidation resistance and adherence of oxide films, however, the use of yttrium has its disadvantages. Yttrium is expensive and is subject to "fade" during melting and pouring of ferrous alloys. Yttrium, because of its highly reactive nature, combines with other elements such as oxygen and is lost to the slag and furnace refractories. Generally, because of the highly reactive nature of yttrium, a more costly process of vacuum induction melting is used for producing iron-chromium-aluminum alloys containing yttrium. Furthermore, during vacuum melting and casting, recovery of yttrium in the metal may typically be less than 50% of that added to the melt composition. If there are any delays or problems which would prevent immediate pouring of the melt, recovery may be substantially lower. Moreover, even vacuum induction melting is inadequate for substantial recovery of yttrium through the remelting of the scrap of yttrium-containing alloys.
U.S. Pat. No. 3,920,583, issued Nov. 18, 1975, relates to a catalytic system including an aluminum-bearing ferritic steel substrate and, particularly, an iron-chromium-aluminum-yttrium alloy. The alloy is disclosed to have the property of forming an adherent stable alumina layer upon the substrate surface upon heating such that the layer protects the steel and makes it oxidation resistant.
To overcome some of the disadvantages of yttrium-containing iron-chromium-aluminum alloys, it has been proposed that other lower cost alloying metals be substituted for yttrium. U.S. Pat. No. 3,782,925, issued Jan. 1, 1974, discloses a ferritic heat resistant iron-chromium-aluminum steel having silicon, titanium and rare earth additions. The alloy contains 10-15% chromium, 1-3.5% aluminum, 0.8-3% silicon and 0.01-0.5% calcium, cerium and/or other rare earths for scale adherence. The patent also requires a total of aluminum and silicon ranging from 2-5%, free titanium of at least 0.2% and a sum of oxygen and nitrogen of at least 0.05%.
An article entitled "High Temperature Oxidation Behavior of Fe-20 Cr-4 Al Alloys With Small Additions of Cerium" by Amano et al, Trans. JIM 1979, Vol. 20, discloses an iron-chromium-aluminum alloy with increasing cerium additions for good adherence of the oxide surface. The article discloses static oxidation tests at cerium amounts of 0.01%, 0.04% and 0.37%. While there was spalling of the oxide coating at the lowest cerium level of 0.01%, no spalling was reported at the higher levels of 0.04% and 0.37% cerium. The cerium existed in the latter two alloys as a Ce-Fe intermetallic compound which precipitated at the grain boundaries. The article does not address thermal cyclic oxidation resistance and hot workability of the alloys.
Other iron-chromium-aluminum alloys containing cerium are known for electrical resistance heating elements. U.S. Pat. No. 2,191,790 discloses up to 5% of an addition chosen from a group of cerium and other elements and further includes up to 0.5% carbon and 0.05-0.5% nitrogen. The objective of the alloy was to improve oxidation resistance, scale adherence and toughness at elevated temperatures greater than 2102.degree. F. (1150.degree. C.). Improvements over the alloy of that patent are shown in U.S. Pat. No. 2,635,164, issued Apr. 14, 1953, and U.S. Pat. No. 2,703,355, issued Mar. 1, 1955.
Japanese Patent Application No. 56-65966, published on June 4, 1981, also discloses an iron-chromium-aluminum alloy having heat absorbing and radiating properties for combustion devices.
It is also known to provide a glass sealing alloy of iron, chromium and aluminum with additions of rare earths up to 2%, disclosed in U.S. Pat. No. 3,746,536, issued July 17, 1973.
There still exists a need, however, for an alloy which is less expensive to produce because of lower cost alloying elements, which can be produced through lower cost melting processes and which is resistant to thermal cyclic oxidation from ambient temperature up to temperatures of about 1600.degree. F. (871.degree. C.), such as in internal combustion exhaust environments, and which has improved hot workability. Furthermore, the alloy should be suitable for providing an improved aluminum oxide surface which is adherent to the metallic surface under thermal cyclic conditions. It is further desired that the alloy be susceptible to further treatment to provide an improved and texturized aluminum oxide surface to provide more surface area and so as to enable more catalytic materials to be supported on the alloy by the aluminum oxide surface.
The alloy should also be capable of being stabilized or, if need by, or being stabilized with elevated temperature creep strength properties improved.